1. Field of the Invention
The present invention relates to imaging, and, more particularly, to a method for providing image illumination calibration for an imaging apparatus.
2. Description of the Related Art
The illumination subsystem is a key factor in the dynamic range and contrast of the image being captured for a camera scan system. FIG. 1 illustrates an uncalibrated image 10 of a white sheet of media, wherein region 12 illustrates illumination from a single point light source. Region 12 represents a ten percent saturation of the total image area, with lens vignetting occurring in regions 14, 16, 18 and 20. Lens vignetting is a reduction in image brightness in the image periphery compared to the image interior.
An ideal camera scan system would include a very uniform distribution of light over the entire area to be captured. In reality, a perfectly uniform light source cannot cost effectively be designed or produced. In traditional scanner systems, the non-uniform light distribution is calibrated out using a method of gains and offsets called shading correction. The function of shading correction is to apply a unique gain to each pixel value so that the resulting image appears to be uniformly illuminated. In essence, the darker pixels are “gained” up to have the same value as the lighter pixels. The lower the uniformity, the higher the gain value needed. As gain values go up, the dynamic range of the system goes down and results in a reduction of gray levels that can be distinguished.
For a full image capture system (e.g., an 8.5 inch by 11 inch image) using an array sensor, the area is larger and harder to uniformly illuminate, especially in a small space. Using multiple light sources improves uniformity, but adds significant cost. Also, the lens reduction required for a full image scan detracts from uniformity due to the vignetting of the lens.